1. Field of the Invention
The present invention relates to a control device for a vehicle, particularly to the control of suppressing occurrence of an impact caused by gear shifting while preventing the event of frequent shifting in the automatic transmission during a constant speed cruising mode of the vehicle.
2. Description of the Background Art
Conventionally in a constant speed cruising mode of a vehicle, the event of frequent shifting may occur due to change in the inclination of the road such as in the case of running on a slope. This frequent shifting refers to the operation of the gear of the automatic transmission repeating an upshift and downshift. In view of the foregoing, Japanese Patent Laying-Open No. 11-078599, for example, discloses a constant speed cruising apparatus that prevents frequent shifting repeatedly between an overdrive (OD) gear and third gear of the automatic transmission. This constant speed cruising apparatus includes an automatic transmission switching a plurality of gears to transmit the power of the internal combustion engine to the wheels, transmission control means, throttle driving means responding to a throttle opening control signal having the duty representing the throttle opening for driving the throttle valve so as to attain the throttle opening represented by the throttle opening control signal, and constant speed cruising control means. The transmission control means shifts the gear of the automatic transmission according to a vehicle speed pattern corresponding to the throttle opening, and derives a transmission signal representing a downshift when down shifting from the overdrive gear to a gear of higher reduction gear ratio. When an overdrive cut request signal is received, upshift return to the overdrive gear is not effected. When an upshift return signal is received, upshifting to the overdrive gear is effected. The constant speed cruising control means provides a throttle opening control signal to the throttle driving means such that the speed of the vehicle corresponds to the set speed, and responds to a transmission signal to derive and provide to the transmission control means an overdrive cut request signal. The constant speed cruising control means further includes throttle opening output means for obtaining the throttle opening, and setting means for setting an opening set value ci for upshift return that is substantially equal to a predetermined throttle opening in a constant speed cruising mode on a flat road. When the throttle opening obtained by the throttle opening output means moves in the closing direction for at least the upshift return opening set value ci, the overdrive cut request signal is canceled and the generated upshift return signal is provided to the transmission control means.
In accordance with the constant running apparatus disclosed in the aforementioned publication, the throttle opening during a constant speed cruising mode in third gear will not operate in a closing direction for more than the set value ci, i.e. the throttle opening will take a large value exceeding ci, when climbing a long upward slope. Therefore, the overdrive cut request signal will not be canceled. Accordingly, the possibility of upshifting from third gear to overdrive gear is suppressed, preventing frequent shifting.
However, when the vehicle speed is low during downshifting in a constant speed cruising control mode, a request is made to increase the throttle opening in order to attain the preset speed. Thus, there is a problem that an impact occurs at the vehicle caused by the increase in the request of the throttle opening when down shifting is completed.
Since the engine output is not transmitted to the driving wheel during down shifting in a constant speed cruise control mode, the speed of the vehicle will become lower. At this stage, the request value of the throttle opening is increased in order to maintain the preset speed. Therefore, control is effected so as to increase the throttle opening. This entails the problem of occurrence of an impact at the vehicle caused by an abrupt acceleration of the vehicle when down shifting is completed at the transmission and the output of the engine is transmitted to the driving wheel.
The constant speed cruising apparatus disclosed in the aforementioned publication may encounter the problem set forth above since increase in the requested amount of throttle opening during down shifting is not taken into account.